"zee" or "zed"?

"zed": There is a famous line in Pulp Fiction, where Butch's girlfriend asks him whose chopper is it, to which he replies "Zed's". She asks him: "Who's Zed?". He replies: "Zed's dead baby, Zed's dead". It is strange for an American film to talk of "zed", where "Zee would have seemed more logical: "Zee's dead baby, Zee's dead". The letter Z is "zed" in British English and "zee" in American English. So, the rapper Jay-Z ("Jay-Zee") should technically be called Jay-Zed in Britain -- which of course he's not.

zeitgeist

Make sure not to spell it with a 'ziet-'. It is often used with italics.

zenith or nadir?

In the non-astronomical sense, zenith is used to mean high point or top, and nadir is used to mean low point or bottom.

zero or nil?

With sport, this varies. Soccer, for example, scores are "2-nil" not "2-zero"..

zeroes or zeros?

zero: This is a tricky little word. First of all, the preferred spelling of the plural is 'zeros' whereas the verb (to 'zero in' on something) is 'zeroes', which is conjugates to 'zeroed' and 'zeroing'. Also, it is common practice in British English to call out a string of numbers (such as a phone numbers etc) by saying "oh". For example, the serial number EA2500670 would, in British English, be read out over the phone: "E-A-two-five-oh-oh-six-seven-oh". Americans are more like to say (and understand) "E-A-two-five-zero-zero-six-seven-zero" and might confuse "oh" for the letter O. Some people object to the perfectly acceptable use of "oh" because they argue that it is a number and not a letter. But in actual fact, the "oh" is short for 'o, from zero. The best known example of "oh" is 007.

zigzag or zig-zag?

zigzag: No hyphen.

zoology

The days when zoology was pronounced exclusively as "zoh-ology" are over, it is perfectly acceptable and not incorrect to say it as "zoo-ology".